This invention relates to a method and apparatus providing for improved curing of lenses, especially heat curing of contact lenses.
Known methods for casting contact lenses include methods referred to in the art as static cast molding. Such methods involve charging a lens-forming monomer mixture to a two-part mold assembly, the assembly including a posterior mold for forming the posterior contact lens surface and an anterior mold for forming the anterior contact lens surface with a mold cavity defined between these two mold surfaces. Then, the monomer mixture is cured while in the mold assembly, and subsequently, the contact lens is recovered from the mold assembly. A common manner of curing the monomer mixture involves heat curing, where the monomer mixture is exposed to heat while in the mold cavity of the mold assembly to effect polymerization of the monomers in the lens-forming mixture. As an example, U.S. Pat. No. 5,143,660 describes a method for cast molding contact lenses in a two-part mold where the molds including the lens-forming mixture are charged to an oven for heat curing the lens-forming mixture in the mold assemblies.
Where it is desired to heat cure the lens-forming monomer mixture, the molds may be supplied to a heating apparatus such as an oven. Often the heat-cure process will be a batch or semi-batch process, in which case it may be desirable to cure multiple lenses concurrently. One approach to increasing the number of lenses that can be heat cured concurrently is to stack the molds in the heating apparatus, for example, a plurality of two-part molds, each containing monomer mixture, can be arranged on a plate, and then multiple plates can be stacked vertically one on top of the other. However, stacking of the plates on top of one another may lead to different external pressures being applied to different molds in the stacked arrangement, for example, molds in the lower portion of the stack receive greater weight than molds in the upper portion of the stack. Additionally, such stacking also results in different temperature profiles among the various molds. Ultimately, this affects consistency of the casting operation, for example, inconsistent dimensions or curing of the lenses.
The present invention solves this problem by providing a method and an apparatus that avoids uneven weight distribution on stacks of mold parts during curing of monomer mixture contained in the molds. The invention also provides for more even heat distribution among the various stacked molds in the heating apparatus.